Zinc electroplating



United States Patent 3,005,759 ZlNC ELECTROPLATING William H. Safranek and Hugh R. Miller, Columbus,

Ohio, assignors, by mesne assignments, to American ginc Institute, New York, N.Y., a corporation of New ork No Drawing. Filed Apr. 24, 1959, Ser. No. 808,555 4 Claims. (Cl. 204-55) This invention relates to zinc electroplating, and particularly to a method and compositions for electroplating bright, leveling zinc.

One object of the invention is to provide a method for eliminating or reducing the costs of polishing and bufling of die castings prior to electroplating with copper, nickel, and chromium. Heretofore, zinc die castings had to be polished and buifed by costly mechanical procedures to eliminate scratches, pits, cold shut, and other surface irregularities to make their surfaces sufficiently smooth for producing a mirrorlike finish by plating with copper, nickel, and chromium. Although several leveling-type copper and nickel plating processes are employed commercially, they do not result in a satisfactory mirrorlike appearance unless the die cast surfaces are first smoothened mechanically. However, by zinc plating with the process of this invention, a smooth bright finish can be obtained on zinc die castings Without the need for polishing or butting them. When the zinc-plated die castings are subsequently electroplated with leveling copper and nickel, and finally with chromium, an excellent mirrorlike appearance is obtained.

Another object is to provide a method for improving the corrosion resistance of electroplated zinc die castings. It has been established that surface pores in die cast surfaces seldom are removed completely by mechan ical polishing or bufling, and it has been established further that such pores, which usually are neither filled nor bridged by copper, nickel, and chromium plating, are the focal points for corrosion when electroplated die castings are exposed outdoors. However, by zinc plating according to this invention, surface pores are filled or bridged with zinc plate. Copper, nickel and chromium then can be electroplated with no discontinuities or pits, thereby improving the corrosion resistance of the electroplated die castings. I

A further object of this invention is to provide a method for improving the appearance of zinc-plated steel parts. Although several processes are employed commercially for electroplating bright zinc, they have little or no leveling power and, as a result, steel parts stamped from hot-rolled steel and other parts fabricated from commercial cold-rolled steel with a No. 1 or No. 2 finish have an inferionfrosty or hazy appearance after zinc plating by prior art processes. The appearance of such parts, however, is improved greatly by zinc plating according to the present invention. Surfaces are more nearly mirrorlike and irregularities such as scratches and pits are covered or hidden, because of the exceptional leveling power of the zinc plate deposited by the present process.

Other objects and advantages of the present invention are apparent from the disclosure herein.

It has been found in the present invention that certain critical ranges of composition and operating conditions are necessary to provide optimum polishing of scratches and leveling, especially pore filling.

Table I below lists preferred and useful ranges of bath compositions and operating conditions according to the present invention.

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TABLE I Bath constituents Preferred range Useful range Zine sulfate, 1.3 to 1.6 molar 0.8 to 1.8 molar.

ZnSO4-7l-I20. Ammonium chloride, 0.15 to 0.4 molar Optional up to 1.0

NlllOl. molar. Soldium sulfate, 0.07 to 0.15 molar 0.07 to 0.4 molar.

NazSOr. Boric acid, HsBO3 0.15 to 0.35 molar 011252131 up to 0.7 Citrate ions (citric acid 0.15 to 0.3 molar 0.05 to 0.6 molar.

ailzdlctissodium c ra e Glycerin 0.01 to 0.1 molar Optional up to 0.1

molar. Saecharln 0.08 to 0.12 molar 0.05 to 0.15 molar. p-Tuluene sulfonamide 0.005 to 0.02 molar 0.003 to 0.025 molar. Tergitol 08 1 (wetting 0.5 to 1.0% by volume. 0.3 to 1.5% by agent). volume. pH an. 2.3 to 2.5 2.0 to 3.0. Operating con t ons:

Periodic current 14 tgel8 seconds 4 to 60 d1rect.

reversal cycle 8 to 10 seconds 1 to 40 reverse.

reverse. Cathode current 200 to 250 amp/sq. ft to 350 amp/sq.

degsity (for)ward it. an reverse Temperature 112 to F 110 to F. Agitation air Suflicient to raise Mild to turbulent.

solution level )6 to 1 inch. Work bar 20 to 30 c.p.m.; 2 to 10 to 60 c.p.m.; 1 to 4 inch stroke. 8 inch stroke. Anode current 30 to 60 amp/sq. ft 10 to 100 amp/sq. ft.

density. Voltage (between 6 to 12 volts 6 to 20 volts.

anode and rl t t 25 a 30 minutes 15 to 60 min a in ime o Plating rate 0.002 to 0.004 inch/ 0.002 to 0.004 inch] hour. hour. I Thickness of deposit. 0.001 to 0.002 inch 0.0005 to 0.003 mcb.

1 Tergitol 08 is a 38 percent solution of sodium sulfate derivative of 2-ethy1 hexanol-l The ammonium chloride increases the conductivity of the bath but can be replaced with other highly conducting salts, such as ammonium fiuoborate or other ammonium halides. Boric acid and citric acid are butters. Citric acid and sodium citrate act as aids in promoting leveling power. Glycerin promotes pore filling. Saccharin and p-toluene sulfonamide are the primary brightening agents.

The wetting agent prevents pitting. It can be omitted if the bath is maintained in a substantially pure condition, free from oil, common grease, and metallic impurities such as copper, lead, and cadmium.

The essentials of the bath composition are (a) zinc sulfate in the range of about 0.8 to 1.8 mols per liter (230 to 500 grams per liter), preferably about 1.3 to 1.6 mols (375 to 450 grams per liter); (b) citrate ions in the range of about 0.05 to 0.5 mol per liter, preferably about 0.l5 to 0.3 mol per liter; saccharin in the range of about 0.05 to 0.15 mol per liter (9 to 27 grams per liter), preferably about 0.08 to 0.12 mol per liter (15 to 22 grams per liter); p-toluene sulfonamide in the range of about 0.003 to 0.025 mol per liter (0.5 to 4 grams per liter), preferably about 0.005 to 0.02 mol per liter (0.9 to 3.5 grams per liter); and acid concentration such as to provide a pH in the range of about 2.0 to 3.0, preferably about 2.3 to 2.5.

A bath prepared with sodium citrate and no citric acid needs an addition of sulfuric acid to reduce the pH to within the desired range. A bath prepared with citric acid and no citrate salt needs an addition of sodium or ammonium hydroxide to increase the pH to within the desired range. A bath prepared with approximately An extensive study was conducted on the properties of the plates deposited in the following bath:

(glycyrrhizin) gradually coagulated in the plating bath and became inert.

(4) Improved physical 'pr0perties.0ld brightener sometimes was occluded in the zinc deposit; after plating with copper, nickel and chromium and heating the electroplated zinc die castings, blisters sometimes were noted in the electroplated zinc layer as a result of the thermal decomposition of the traces of the occluded glycyrrhizin. Similar heat tests (up to 350 F.) with die castings plated with zinc by the new procedure and then plated with copper, nickel and chromium showed no Bend tests of zinc-plated steel panels showed a better ductility for the zinc deposited by the new procedure, compared with the zinc deposited by the previous (5) Simplifies plating bath cntr0l.- The previous bath containing 'glycyrrhizin had to be continuously fil- Zinc sulfate, ZnSO -7H O 1.2 mols. Ammonium chloride, NH CL--. 0.08 mol. Sodium sulfate, Na SO 0.05 mol. Boric acid, H BO 0.16 mol. 10 Citric acid 0.26 mol. Glycerin 0.08 mol. bhstenng- Saccharin (soluble sodium salt)- 0.1 mol. p-Toluene sulfonamide 0.016 mol. V Tergitol O8 percent, by volume, 15 Procedure Y Y Sodium hydroxide (to raise the pH to about 2.4) Approximately 10 g./l.

This bath was operated for more than two weeks. Operating conditions during this two-week period were in the following ranges:

Temperature 110 to 118 F. Current density 150 to 300 amp/sq. ft.

Periodic current deversal cycle: Direct for seconds Reverse for 9 seconds tered to remove coagulated glycyrrhizin; filtration is not required for the new procedure, except for removing dirt accidentally introduced into the plating bath from the air above the solution or from parts being plated. By the old procedure, glycyrrhizin had to be added at frequent intervals to maintain bright, leveling plates. By the new procedure, brighteners are added only at infrequent intervals merely to replace consumption by dragout.

What is claimed is: v

-1. A method of bright, leveling Zinc electroplating comprising plating zinc upon a conductive cathode from a zinc solution consisting essentially of about 1.3 to 1.6 mols per liter of zinc sulfate, about 0.15 to 0.3 mol per liter of citrate ions, about 0.08 to 0.12 mol per liter of saccharin, and about 0.005 to 0.02 mol per liter of p-toluene sulfonamide, said solution having a pH from about 2.3 to 2.5; providing the plating by a periodic current reversal cycle alternating between forward current TABLE II EFFECT OF INCREASING THE CATHCDE CURRENT DENSITY 0N APPEARANCE CF LEVELING ZINC PLATE \1 Cathode Plating Rate of Appearance of zinc plate current Voltage, Temperatime, deposiamp/sq. ft. mils/hr, Before plating with copper, nickel, chromium After plating with copper, nickel, chromium 3.4 117-118 2.4 Very slightly grainy in recesses; nonuni- Superior leveling; very slightly hazy at low formly semibright. current density areas. 4.4 116 40 3. 2 No apparent graininess; uniformly semi- Superior leveling; no grammess; mirror l bright except for slightly dull recesses. bright. 5.4 115418 31 3. v No graininess, uniformly semibright; Do. 6.4 120 25 4. 8 No graininess, uniformly hazy bright; very Do.

, slightly burned at top of casting.

' The zinc bath composition was as follows:

113130 Sodium Citrate... Na2SO4' Gly erin Saeolwrih p-Toluene sulfonamidii. Tergitol 08, by volume- Operating conditions:

PR cycle- Dircct for 15 seconds; Reverse for 9 seconds. Agitation: Air plus work bar. Temperature: to T.

b The current density during the reverse part of thecycle was about 5 percent higher than the current density during the direct part of the cycle.

The present process provides several unexpected advantages over the process of the U.S Patent 2,905,603 of William H. Safranek and Hugh R. Miller:

(1) Broader bright plating range.Plate is uniformly bright on recesses as well as high current density areas, whereas plate deposited by previous procedure was dull in recesses.

(2 Better leveling power.

(3) Improved brightener stability.New brighteners for about 14 to 18 seconds at a cathode current density of about 200 to 250 amperes per square 'foot and reverse current for about 8 to 10 seconds at a cathode current density of about 200 to 250 amperes per square foot, while maintaining the temperature of said solution between about 112 and 120 F.

2. A composition of matter for bright, leveling zinc electroplating consisting of an aqueous solution consisting essentially of about 1.3 to 1.6 mols per liter of zinc are consumed only by dragout, whereas old brightener 7 sulfate, about 0.15 to 0.3 mol per liter of citrate ions,

about 0.08 to 0.12 mol per liter of saccharin, and about 0.005 to 0.02 mol per liter of p-toluene sulfonamide, said solution having a pH from about 2.3 to 2.5.

3. A method of bright, leveling zinc electroplating comprising plating zinc upon a conductive cathode from a zinc solution consisting essentially of about 0.8 to 1.8 mols per liter of zinc sulfate, about 0.05 to 0.5 gram per liter of citrate ions, about 0.05 to 0.15 mol per liter of saccharin, and about 0.003 to 0.025 mol per liter of p-toluene sulfonamide, said solution having a pH from about 2.0 to 3.0; providing the plating by a periodic current reversal cycle alternating between forward current for about 4 to 60 seconds at a cathode current density of about 100 to 350 amperes per square foot and reverse current for about 1 to 40 seconds at a cathode current density of about 100 to 350 amperes per square foot, while maintaining the temperature of said solution between about 110 and 130 F.

4. A composition of matter for bright, leveling zinc electroplating consisting of an aqueous solution consist- References Cited in the file of this patent UNITED STATES PATENTS 808,103 Meaker Dec. 26, 1905 2,905,603 Safranek et al Sept. 22, 1959 FOREIGN PATENTS 693,606 Great Britain July 1, 1953 OTHER REFERENCES Bray et al.: Transactions, Electrochemical Society, vol. 78 (1940), pp. 309-316. 

1. A METHOD OF BRIGHT, LEVELING ZINC ELECTROPLATING COMPRISING PLATING ZINC UPON A CONDUCTIVE CATHODE FROM A ZINC SOLUTION CONSISTING ESSENTIALLY OF ABOUT 1.3 TO 1.6 MOLS PER LITER OF ZINC SULFATE, ABOUT 0.15 TO 0.3 MOL PER LITER OF CITRATE IONS, ABOUT 0.08 TO 0.12 MOL PER LITER OF SACCHARIN, AND ABOUT 0.005 TO 0.02 MOL PER LITER OF P-TOLUENE SULFONAMIDE, SAID SOLUTION HAVING A PH FROM ABOUT 2.3 TO 2.5, PROVIDING THE PLATING BY A PERIODIC CURRENT REVERSAL CYCLE ALTERNATING BETWEEN FORWARD CURRENT FOR ABOUT 14 TO 18 SECONDS AT A CATHODE CURRENT DENSITY OF ABOUT 200 TO 250 AMPERES PER SQUARE FOOT AND REVERSE CURRENT FOR ABOUT 8 TO 10 SECONDS AT A CATHODE CURRENT DENSITY OF ABOUT 200 TO 250 AMPERES PER SQUARE FOOT, WHILE MAINTAINING THE TEMPERATURE OF SAID SOLUTION BETWEEN ABOUT 112 AND 120*F. 